Nowadays, optical backbone networks are mainly based on International Telecommunications Union (ITU-T) G.709/G.798 standards, which define an Optical Transport Network (OTN). Such standards allow the transport of a range of different traffic technologies, such as Ethernet and Synchronous Optical Networking (SONET)/Synchronous Digital Hierarchy (SDH). Referring to FIG. 1, an OTN network 4 can be considered as a central cloud connecting peripheral/client networks 1,2,3, which can be based on different technologies. For example, a national optical backbone 4 can be used to connect peripheral regional SDH rings or Ethernet/MPLS sub-networks 1,2,3.
The boundary nodes 10 at the boundary between the OTN 4 and each of the client networks 1,2,3 map the client signals (e.g. STM-n/OC-n, GbE, 10GE, etc.) into Optical Channel Data Unit (ODUk) containers for transport within the OTN 4. Within the OTN 4, the traffic can be protected using standard ODUk Sub-Network Connection (SNC) protection as defined by ITU-T G.873.1.
FIG. 2 shows ODUk SNC protection between two nodes, Node 1, Node 2, of an OTN 4. In one protection scheme, there is duplication of client traffic at Node 1, and the duplicated traffic is sent over two independent paths of the OTN Network 4. A first path (ABD) is called the “working” path and a second path (A→C→E) is called the “protecting path”. The destination node, Node 2, will select the traffic (i.e. ODUk) either from the W path or from the P path depending on quality information. The quality information can be a Signal Fail SF or Signal Degrade SD detected by the traffic cards D and E at Node 2. There is OTN ODUk switching fabric in each of Node 1 and in Node 2.
Each client network 1,2,3 connected to the OTN 4 has switching capability at the client layer, which can be used for the purpose of providing protection within the client network. A boundary node 10 can be provided with both a client network switching fabric (e.g. at the SDH VC-n layer) to close protections at the SDH layer and an OTN switching fabric (e.g. at the ODUk layer) to close protections at the OTN layer. An example of a node with two switching fabrics is shown in FIGS. 3 and 4. In FIG. 4, the client traffic is SDH. Starting from the left-hand side of FIG. 4, a VC-n Sub-Network Connection Protection (SNCP) is closed in the SDH fabric, then the result of the protection is mapped into an ODUk. An ODUk Sub-Network Protection is opened by the ODUk fabric of the node. This arrangement is effective, but it has a disadvantage of being expensive to implement because the node must be provided with two switching fabrics, one for the client network and one for the OTN.